Wheat

Chemistry and Utilization

Hugh Cornell, Albert W. Hoveling

• Editorial: CRC Press
• Año: 2020
• ISBN:
  9781000725445
• Edición: 1
• Categoría: Biological Sciences
• Categoría: Environmental Science

"This book meets the need for a comprehensive, up-to-date review of wheat chemistry, processing and uses. It provides the reader with extensive new information on wheat components that will be useful in better commercial utilization of wheat and the formulation of new and upgraded wheat-based food products. The book serves as a one-volume information resource for all those involved in the research, development, formulation, and evaluation of wheat-based food products. From the Authors' PrefaceWheat continues to be one of the world's most important grains, especially as a food, where the unique properties of its products can be utilized to advantage. It provides an excellent example of a natural product from which a wide range of useful by-products can be made. This book discusses the components of the wheat kernel, which provide interesting examples of study of carbohydrate and protein chemistry, as well as lipids, minerals and vitamins.
This book should serve as a useful reference for the cereal chemist, as well as chemists and food technologists in those industries in which by-products of flour are used, e.g., the confectionery industry in which modified starches and starch syrups are used. In addition, nutritionists, dieticians, and many kinds of researchers will find chapters of interest. Particular attention is given to particle-size determinations, an important area in food processing, and to the role of wheat proteins in gluten intolerance and wheat allergy.... Both the milling of wheat and flour quality are discussed in order to give the reader an idea of the distribution of the major components and the importance of proper size reduction. The book also has a chapter on wet milling of wheat flour... and chapters on the properties and uses of wheat starch, starch syrups, and chemically modified wheat starch.

CHAPTER 1

The Wheat Kernel

1.0 INTRODUCTION

WHEAT is a wild grass (Gramineae family) native to the arid countries of western Asia. Its use as a food goes back to the Stone-Age era. Altogether, about 600 genera of grasses have evolved, among them the various forms of the genus Triticum, of which the following are the main groups

aestivum (vulgare)—Common wheat

durum—Durum wheat

compactum—Club wheat

turgidum—Poulard wheat

dicoccum—Emmer wheat

spelta—Spelt wheat

polonicum—Polish wheat

For commercial purposes, wheat is generally classified as hard or soft, red or white, spring or winter so that, for example, we refer to a wheat as being a “hard red winter wheat.”

Wheat and related grasses such as barley and rye have always been important for food in Europe, the Levant, and the western part of Asia. In Roman times, barley was the most important of these cereals, but by the Middle Ages it had been replaced to a large extent by rye, especially as the latter became important as animal fodder.

In later years, wheat became the leading cereal crop as by then bread, especially the leavened varieties, had become an important part of the daily diet. Wheat was the only grain suitable for leavened bread. This is due to the presence of a unique elastic protein complex (the gluten complex) that provides a matrix for the gases to form the characteristic open texture of this bread. Flat or unleavened bread is still popular in the Levant and western Asia. In many African countries, eating habits have changed in favor of wheat bread and away from cereal pap, particularly since the 1970s, when food aid in the form of wheat was introduced (Steller 1993).

Today, large quantities of wheat are produced in Europe, Pakistan, India, China, South and North America, Australia, and several other countries. Some of the best quality wheat is grown in Canada. Altogether, about 600 million tonnes of wheat are produced around the world, ahead of rice, corn, sorghum, rye, barley, oats, and millet. Buckwheat, a herbaceous plant, and soybeans, a legume, are not classed as cereals.

Many new varieties of wheat have been obtained through plant-breeding programs. The dwarf wheats—mainly Japanese and American hybrids—have been successfully grown in Mexico, India, and Pakistan, where climatic conditions make high yields of conventional wheats difficult to obtain. Other new varieties have been introduced in order to maximize disease-resistant properties and to obtain the best baking quality for the particular market targeted.

Grain quality is generally assessed by texture (hardness), color, content of foreign matter, percentage of broken kernels, moisture content, and baking quality. In order to test the latter, some form of test milling is required and the flour so produced must be evaluated for its doughing properties, as well as the final baked product. The milling of wheat is discussed in Chapter 2.

Triticale is a cross of wheat (Triticum) and rye (Secale) used for animal feed. It has a better balance of amino acids than either of the parent grains, and the absence of a sticky dough ball (unlike wheat) is a decided advantage in this application.

1.1 THE STRUCTURE OF THE WHEAT KERNEL

The wheat kernel or grain, known botanically as a caryopsis, is the fruit of the plant and is normally about 4–8 mm long, depending on the variety and condition of growth. The kernel contains only one seed, which is not shed at maturity, in common with other grasses. The color of the kernel is governed primarily by materials present in the seed coat or pericarp (Figure 1.1). This consists of an epidermis (outer layer) and a hypodermis, next to a layer of thin-walled cells and several other types of cells. Altogether, this pericarp is about 50 μm thick. Then we find another thin seed coat, covering a nucellar epidermis, and then an aleurone layer, before coming to the starch-rich endosperm (Figure 1.2). Bran is chiefly the outer material down to and including the aleurone layer.

Figure 1.1 Longitudinal view of the wheat kernel. (Reprinted from Food Chemistry, 2nd Ed., edited by Owen R. Fennema, p. 875, by courtesy of Marcel Dekker Inc.)

The starchy endosperm is the material from which white flour is made. It comprises starch granules embedded in a matrix of proteins. The proteins consist of albumins, globulins, gliadins, and glutenins. The combination of the gliadins and glutenins is referred to as the “gluten complex” and is regarded as storage protein. It is formed in discrete particles by proteoplasts, which are able to be seen in developing kernels, and can be separated from the interstitial protein. The biosynthesis of proteins is discussed in Chapter 7.

The starch is present as lenticular granules varying in size from 10 to 50 μm diameter (major axis) and smaller, more spherical granules of 2–5 μm diameter (Sandstedt 1946). More is said about the size distribution of the starch granules in Chapter 3. The starch granules also contain protein and lipids as minor constituents and the amounts are related to the size of the granules. These aspects are also discussed in Chapter 3. The production of starch in the wheat plant by photosynthesis is described in Chapter 4.

Figure 1.2 Cross-sectional view of the wheat kernel. (Reprinted from Food Chemistry, 2nd Ed., edited by Owen R. Fennema, p. 875, by courtesy of Marcel Dekker Inc.)

The wheat germ consists of several parts. The plumule, which forms the shoot when the seed germinates, has a stem attached to it and to the coleoptile, which functions as a protective sheath. There is also the scutellum, the storage, digestive, and absorbing organ, which is attached to the plumule. It contains food for the plant, which is supplied at germination, and also transfers further food from the endosperm. The germ is readily separated from endosperm and bran by milling. It is an important dietary supplement, providing a rich source of vitamin E.

1.2 WHEAT PRODUCTION

As mentioned previously, wheat can be classified as spring or winter type. The first type is planted in the spring and is harvested in the late summer. Yields of spring wheats are usually lower than winter wheats because the latter are planted in late summer or the fall (autumn) and are able to make more effective use of sunshine and moisture. Provided that temperatures are not low enough to kill the plant, the economics of the winter wheats are the more favorable of the two.

When it comes to quality of the wheat, however, hard spring wheats such as those grown in Canada and the United States are among the best. Wheats of this type, such as Manitoba and Dark Northern Spring, have been marketed for many years. Canadian-grown wheat is almost entirely hard spring wheat and the top grades set an extremely high standard for wheat producers.

1.2.1 YIELDS

The grading and blending of different types of wheat are very difficult undertakings. Seasonal and environmental variations can be considerable and make it difficult to predict the quality of any new crop. Added to this problem is the uncertainty of yield. Some countries do not attempt to segregate wheat by type or quality, which is reasonable as long as the quality range is not wide. However, quality differs considerably in most countries due to the environmental factors and the variety of wheat sown, as well as the way in which the wheat is stored and handled.

Common wheat (Triticum aestivum) is the most widely cultivated species of wheat. Club wheat (T. compactum) and durum wheat (T. durum) are species grown on a much smaller scale.

The variety of wheat can affect the yield significantly because the selection for high protein content usually means a reduction in yield (Terman et al. 1969, O’Brien et al. 1989), hence selection has to be made with both protein content and yield in mind. It is therefore common practice to calculate the yield also as kg N/hectare. The use of legumes, plants that “fix” nitrogen from the air and thus build up soil nitrogen, is essential for good yields of N/hectare.

Continuous cultivation of wheat with little or no use of nitrogen-rich fertilizers will result in a reduced ability of the soil to produce high-protein grain. There are certain cultivars that can be grown to increase the protein content of the wheat, but optimal balance between protein content and yield can only be achieved by the use of fertilizers. Low grain-protein figures (<10%) can be an indication that the soil is low in nitrogen, and the best way to correct such deficiencies is by the use of pasture and crop rotations so that fields of wheat are fallow for a season or two (Lawrence 1991). Lucerne, legume, or clovers are all used for the purpose of obtaining sustainable production of wheat with higher grain protein in good yield.

Obviously the selection of suitable sites for wheat growing is also an important consideration. Factors such as soil type, moisture levels, nutrient content, and weather are of prime importance.

Yield is also affected by the variety of wheat insofar as a high resistance of that variety to insect attack and disease will enhance the yield. The most prevalent disease is rust. Rusts are fungal infections caused by the Uredinales, a large order of parasitic fungi in the subclass Heterobasidiomycetes. They produce haustoria that penetrate the living cells of the host. Smut and Bunt parasites are Basidiomycetes, which form a parallel order to the Uredinales. The mycelium grows and produces its spores that attack the wheat in the flower.

1.2.2 BENEFITS OF TRACE ELEMENTS

New developments in agronomy have led to better yields not only through supply of basic nutrients in optimal supply, but also through the supply of essential trace elements (micronutrients). It has long been recognized that those metal ions connected with photosynthesis (magnesium, iron, and manganese) must be added to soils that lack minerals containing these elements.

More recently, micronutrients such as copper, zinc, boron, and molybdenum have been shown to be helpful on certain soil types that are deficient in such elements. These elements are activators of plant enzyme systems.

Research in the People’s Republic of China has indicated that applications of small amounts of rare-earth elements have brought about increased yields of a number of crops. The researchers have claimed that rare-earth elements can increase nutrient uptake of some crops, affect certain plant enzymes, and increase the rate of photosynthesis....